A renewed assault is being made on the popular idea that the mass extinction of large mammals in North America around 10,500 years ago was the result of human hunting.

The overkill hypothesis was first put forward more than a century ago and has been widely accepted for the past 30 years. But it does not square with the known facts and has become more a faith-based credo than good science, said Donald Grayson, an archaeologist at the University of Washington.

Understanding what caused the extinction has implications for conservation biology.

Grayson, who specializes in vertebrate paleontology and archaeology, argues that a call by some environmentalists to return Ice Age mammalselephants, camels, llamas, and other large herbivoresto the southwestern United States is based on bad science.

"Overkill proponents have argued that these animals would still be around if people hadn't killed them and that ecological niches still exist for them," said Grayson. "Those niches do not exist. Otherwise the herbivores would still be there."

Grayson points to climate shifts during the late Pleistocene and related changes in weather and vegetation patterns as the likely culprits in the demise of North America's megafauna.

Islands and Continents

The number of large mammals that became extinct in North America at the end of the late Pleistocene, about 10,500 years ago, is staggering. Among some 35 different kinds of animals that disappeared from the fossil record were mammoths, mastodons, camels, horses, giant ground sloths, and bears.

A leading proponent of the overkill theory, Paul S. Martin, believes the Ice Age megafauna disappeared not because they lost their food supply but because of human hunting.

The extinction of animals as a result of human colonization in island settings has been well documented and the causes widely agreed on.